Since fuel economies and the need to preserve the environment have become priorities, it has become desirable to produce mixes having good mechanical properties and as low a hysteresis as possible so that they can be processed in the form of rubber compositions usable for the manufacture of various semi-finished products involved in the constitution of tires, such as treads, and in order to obtain tires having reduced rolling resistance.
Among the numerous solutions proposed for reducing the hysteresis of tread compositions and, consequently, the rolling resistance of tires comprising such compositions, mention may for example be made of the compositions described in patent specifications U.S. Pat. No. 4,550,142, U.S. Pat. No. 5,001,196, EP-A-299 074 or EP-A-447 066.
An attempt has also been made to improve the grip of the tires using tread rubber compositions incorporating specific plasticizers.
European patent specification EP-A-1 035 164 thus describes a rubber composition for a tire tread which is intended to improve the grip on dry ground of the tire incorporating it. To this end, this composition comprises a liquid elastomer consisting of a polybutadiene the vinyl linkage content of which varies from 40 to 95%, the number-average molecular weight of which may attain 20,000 g/mol and the glass transition temperature Tg of which may vary from −40° C. to −5° C. This composition furthermore comprises a plasticizing resin the softening temperature of which varies from 30° C. to 200° C.
U.S. patent specification U.S. Pat. No. 6,204,320 also describes a rubber composition for a tire tread which is intended to improve the grip of the tire incorporating it. This composition comprises a liquid elastomer consisting of a copolymer of isoprene and of butadiene, the number-average molecular weight of which may reach 50,000 g/mol and the glass transition temperature Tg of which may vary from −50° C. to 20° C. This composition furthermore comprises an aromatic plasticizing oil and a plasticizing resin.
U.S. patent specification U.S. Pat. No. 4,866,131 also describes a rubber composition for a tire tread intended to improve the grip of the latter, comprising a copolymer of an aliphatic diene and a vinylaromatic monomer the number-average molecular weight of which may also reach 50,000 g/mol.
In addition to this reduction in the rolling resistance and this improvement in grip, it is equally desirable to improve the wear resistance of the tire treads and, consequently, to increase the life of the latter (this improved wear resistance also having the effect of reducing over time the debris of tires on the ground due to running and the quantity of worn tires which are sent for recycling, which helps to preserve the environment).
Relatively few solutions have been proposed to date to improve this wear resistance. Mention may be made, for example, of the compositions described in patent specifications JP-A-61 238501, EP-A-502 728 or EP-A-501 227. For it is well-known to the person skilled in the art that an improvement in one performance type for tires is frequently obtained to the detriment of the other performance types.